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Voltage Drop Calculator

Korean Electrical Code formula e = (k × L × I) / (1000 × A) with auto distance-based limit comparison (3% / 5% / 6% / 7%).

Circuit

Cable

Result

Enter circuit information and click Calculate.

What this tool does

The Voltage Drop Calculator takes conductor cross-section, current, run length, and phase type, then computes the voltage drop (V) and drop percentage (%) and compares it against the distance-based allowable limits in Korea's KEC wiring standard (formerly the Naeseon code). It supports both copper and aluminum conductors and applies the correct coefficient for single-phase 2-wire, single-phase 3-wire, 3-phase 3-wire, and 3-phase 4-wire circuits. Use it to confirm whether a wire gauge is adequate for the run distance and whether the voltage at the far end stays within equipment tolerance.

Who uses this

  • Check whether the drop exceeds the limit when a load is far from the distribution panel
  • Decide whether to step up to a larger wire gauge because of excessive drop
  • Compare voltage drop between single-phase and 3-phase wiring options
  • Assess the extra drop when using aluminum conductors instead of copper
  • Learn the voltage-drop formula for Korean electrician licensing exams

How to use (4 steps)

  1. 1Enter the line (or phase) voltage (V) and load current (A) — e.g., 220V single-phase, 380V 3-phase.
  2. 2Enter the run length (m, one-way) — the actual wire length from the panel to the load.
  3. 3Select the conductor cross-section (㎟) from the KS standard sizes, the conductor material (copper/aluminum), and the phase type (single-2, single-3, 3-phase-3, 3-phase-4).
  4. 4Read the voltage drop (V), drop percentage (%), the distance-based limit, and the pass/fail verdict. If it fails, increase the cross-section or shorten the run.

Formula & basis (KEC / former Naeseon code)

■ Voltage drop e = (coefficient × L × I) ÷ (1000 × A) L = run length (m, one-way), I = current (A), A = cross-section (㎟) ■ Coefficient by phase type (copper) Single-phase 2-wire 35.6 3-phase 3-wire 30.8 Single-phase 3-wire / 3-phase 4-wire 17.8 Aluminum conductor: coefficient × 1.6 ■ Drop percentage e(%) = (e ÷ V) × 100 ■ Allowable drop by distance (combined feeder + branch) ≤ 60m — 3% / ≤ 120m — 5% / ≤ 200m — 6% / > 200m — 7%

Worked examples

Example 1: 220V single-phase, 20A, 50m, 4㎟ copper

e = (35.6 × 50 × 20) ÷ (1000 × 4) = 8.9V. Drop = 8.9 ÷ 220 × 100 ≈ 4.05%. The ≤60m limit is 3%, so this fails. Increasing to 6㎟ or larger brings it down to about 2.7% and passes.

Example 2: 380V 3-phase 3-wire, 30A, 80m, 10㎟ copper

e = (30.8 × 80 × 30) ÷ (1000 × 10) = 7.39V. Drop = 7.39 ÷ 380 × 100 ≈ 1.95%. Within the ≤120m limit of 5%, so it passes.

Example 3: switching from copper to aluminum

Same as Example 1 but aluminum: coefficient 35.6 × 1.6 = 56.96 → e ≈ 14.24V, drop ≈ 6.47%. Aluminum's lower conductivity raises the drop by about 60% at the same gauge — you must size up at least one step.

Frequently asked questions

Is the run length one-way or round-trip?

Enter the one-way length (panel to load). The return current path is already accounted for in the phase coefficients (35.6, 30.8, 17.8), so do not double the distance.

What are the 3% / 5% allowable limits based on?

They come from Korea's KEC wiring standard (formerly the Naeseon code): 3% for runs ≤60m, 5% for ≤120m, 6% for ≤200m, 7% over 200m. These are end-of-run values combining feeder and branch circuits, set to keep lighting and equipment within their normal operating voltage.

What should I do if the drop exceeds the limit?

Either (1) step up the conductor cross-section, (2) shorten the run by relocating the panel, or (3) switch single-phase to 3-phase (lower coefficient). Increasing the wire gauge is the most common fix.

Why does aluminum wire have a larger voltage drop?

Aluminum's conductivity is about 61% of copper's, so at the same cross-section its resistance — and voltage drop — is roughly 1.6× higher. This calculator applies a ×1.6 factor automatically when aluminum is selected. To match copper's drop, size aluminum one or two steps larger.

Does it account for DC or power factor?

This is a simplified AC calculation assuming a power factor of 1, ignoring line reactance (inductance). Long, high-current, or low-power-factor circuits are more affected by reactance — use a licensed engineer or design software for precise work.

Cautions

  • •Simplified AC calculation assuming power factor 1 and ignoring reactance. Precise design needs full line-impedance calculation.
  • •Enter the one-way length; the round-trip path is already in the phase coefficient.
  • •Allowable limits follow the KEC distance bands; actual design depends on feeder/branch allocation and equipment characteristics.
  • •Reference calculation only. Have an electrical safety manager or qualified engineer review actual installations.
  • •Cross-section must be a KS standard size.

Last reviewed: 2026-06-17

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